1. Technical Field of the Invention
This invention relates to an energy trapping ceramic resonator wherein mechanical vibration energy is concentrated to a part of a piezoelectric ceramic resonator and a ceramic filter using said ceramic resonator.
A ceramic resonator using a piezoelectric ceramic material provides many advantages such as small size, no adjustment and, coil etc. and therefore, is widely used as the intermediate frequency filter of radio communication equipment. Moreover, with excellent alignment with IC's and LSI's and advancement in the performance of recent piezoelectric ceramic materials, these ceramic resonators will expand their applicable fields in the future.
The ceramic resonators being placed in practical use can be classified into two groups; those utilizing contour vibration, such as radial mode vibration, and the so-called energy trapping type, utilizing the thickness (or width)-extensional vibration or thickness (or width)-shear vibration. However, these ceramic resonators are hard to fabricate for the frequency range from several hundred kHz to several MHz and becomes large for such a frequency band, thus delaying the realization of practical use. Recently, a new type resonator, different from the conventional ones, (the conventional one being the edge mode resonator, the thickness shear resonator, and the thickness-extensional resonator) has been noted for use as a ceramic resonator used in the bandwidth from several hundred kHz to several MHz.
A ceramic resonator using the thickness extensional vibration of a rectangular shape piezoelectric ceramic plate can be manufactured very easily because the polarizing direction and the AC field direction for activating vibration are identical. Moreover, since a small energy trapping type resonator can be realized for the frequency range from several hundreds kHz to several MHz, it is suitable as a ceramic resonator in the above-mentioned frequency band which has conventionally been considered difficult to obtain.
2. Description of the Prior Art
A resonator utilizing contour vibration, such as the radial mode vibration, can be made smaller as its frequency range becomes higher, because the resonant frequency is reversely proportional to dimensions of its contours or to the diameter in the case of a disk, thus making manufacturing difficult. Therefore, the upper end of the practical frequency range is limited to several hundreds kHz. Moreover, in a resonator utilizing contour vibration, the entire body vibrates and it is only at the node that the vibration displacement becomes zero; the node is located at the center of the disk in the case of a radial mode vibration. For this reason, a resonator is supported at the node. When a supporting means at the node is made more rigid, energy leaks to the outside through the supporting means and, therefore, a comparatively soft supporing means is generally employed. As a result, reliability to external vibration is lowered.
FIG. 1A is a perspective view of a resonator utilizing the above-mentioned radial mode vibration. In this figure, the thin film electrodes 62, 62' are formed by evaporation on both sides of the piezoelectric ceramic platelet 61. This resonator is supported in such a manner as shown in FIGS. 1B, C. In FIG. 1B, one end of a thin wire 63 is fixed to the metal supporting post 64 which is in turn clamped to the insulation substrate 66. The wire affixed, with conductive adhesive, to the center of the metal thin film 62 on both sides of the piezoelectric ceramic platelet 61. In FIG. 1C, a metal supporting means 65, fixed to the insulation substrate 66', is composed of an elastic material and the supporting means 65 is connected, at its end portion, to the center of metal thin film electrodes 62, 62' on both sides of the piezoelectric ceramic platelet 61, in order to hold the resonator 6.
On the other hand, a resonator utilizing the thickness-extensional vibration or thickness-shear vibration of a piezoelectric ceramic platelet is capable of assuring high reliability to external vibration. This is because the resonant frequency is reversely proportional to the plate thickness and vibration energy is trapped at the center of the plate, therefore, the external edge of the piezoelectric ceramic platelet can be held firmly. However, the external dimension of the resonator is set at a value of greater than 30 times the plate thickness in order to improve the vibration energy trapping performance.
For example, a resonator for the frequency of 10 MHz has a thickness of about 0.13 mm and a diameter of about 3 mm, but a resonator for the frequency of 1 to 2 MHz has a thickness of 0.7 to 1.3 mm and a diameter of 20 to 30 mm, the increase in size being a feature of ceramic resonators. For this reason, ceramic resonators of the energy trapping type have been put into practical use only in the frequency of several MHz or more.
FIG. 2 is a perspective view of an example of a ceramic resonator 7 utilizing thickness-extensional vibration, wherein the metal thin film electrodes 72, 72' are formed by evaporation at the of both sides of the piezoelectric ceramic platelet 71; The edge of the piezoelectric ceramic platelet 71 is held by conductive metal holders 74, 74', which are connected to the metal thin film electrodes 72, 72' via the connecting belt electrodes 73, 73'. A greater part of the vibration energy is trapped within the area sandwiched by the opposing metal thin film electrodes 72, 72' with this configuration.
As explained above, the conventional ceramic resonator has the disadvantages that it becomes physically too small for the frequency of several hundreds kHz to several MHz, making manufacturing difficult or it becomes too large mismatching an LSI circuit etc. Moreover, a ceramic resonator of the contour vibration mode has not yet been put into practical use for high frequency bands because the reliability of the supporting means is insufficient.
A usual piezoelectric ceramic material used as a ceramic resonator has a Poisson's ratio of 1/3 or less. Therefore, it is known that the ceramic resonator which consists of a rectangular piezoelectric ceramic platelet shows thickness-extensional vibration functions as a backward-wave-mode energy trapping resonator. The relation between the resonant frequency f.sub.0 of extensional vibration and the size of this ceramic resonator is expressed by the equation (1). EQU f.sub.0 ={1/(2W.sub.0 }.v[Hz] (1)
Where, W.sub.0 is width of resonator [m] and v is extensional propagation velocity [m/s].
FIG. 3 is a perspective view indicating the conventional structure of a ceramic resonator utilizing the thickness-extensional vibration of a piezoelectric ceramic platelet.
In this figure, 81 is a piezoelectric ceramic platelet, with a pair of thin film electrodes 82, 82' attached by evaporation etc. at the centers of the longitudinal direction. This ceramic resonator is fixed at its edge, in the longitudinal direction of the piezoelectric ceramic platelet, by a holding means not indicated, and an AC electric field is applied in order to drive the ceramic resonator via said holding means. Therefore, said electrodes 82, 82' are respectively provided with strip electrodes 83, 83' which are mutually extending toward different edges, as shown in this figure. The arrow mark 84 shows the polarization direction.
When an AC electric field is applied across the electrodes 82, 82', width-extensional vibration is excited by the piezoelectrical effect of the piezoelectric ceramic platelet. The ceramic resonator of FIG. 3 is an energy trapping type resonator based on the complex branches (Reference: US 78-63, Mar. 28, 1978, The Institute of Electronics and Communication Engineers of Japan, Technical Group on Ultrasonic) and the width-extensional vibration is trapped within the piezoelectric ceramic platelet area to which a pair of opposing electrodes are attached.
A ceramic resonator of the width-extensional vibration mode is thus realized and the manufacturing of the ceramic resonator shown in FIG. 3 will now be explained. First, the piezoelectric ceramic platelet 82 is of a specified size finished by grinding, etc., then thin film partial electrodes 82, 82' must be attached, for example, with the evaporation, to the main surfaces in the thickness direction as shown in FIG. 3. In order to obtain the partial electrodes, it is necessary to place a mask on the piezoelectric ceramic platelet 81 so that electrode is not attached to the unwanted area. But in this case, for the frequency of 2 MHz the piezoelectric ceramic platelet 81 measures about 0.2 mm in the thickness, about 0.9 mm in the width and about 15 mm in the length, therefore, it is very difficult to accurately place the mask within such dimensions. Upon completion of the electrode attaching operation, a polarization direction 84 is created by appying a DC voltage across opposing electrodes, thus a piezoelectric ceramic resonator 8 be obtained. Moreover, this ceramic resonator must be subjected to an adjustment of the resonant frequency in order obtain the desired characteristics.
Since the resonant frequency of width-extensional vibration is reversely proportional to the width of the resonator, the width of piezoelectric ceramic platelet is reduced by grinding etc. in order to adjustment the frequency. A ceramic resonator is usually required to have a frequency adjustment accuracy of 1 to 5.times.10.sup.-, and the frequency is adjusted with the accuracy of 200 Hz to 1000 Hz for above ceramic resonator. Obtaining a grinding accuracy of 0.09 .mu.m to 0.45 .mu.m for the 2 MHz ceramic resonator which is required to have the above accuracy of grinding of the width dimension, is very difficult from the point of view of current grinding technique.
As explained above, an existing width-extensional resonator provides the feature of realizing a small size ceramic resonator for the frequency of several hundreds kHz to several MHz but is not economical because of the time required to manufacture and adjust the ceramic resonator by grinding of a high accuracy.
Only one ceramic resonator is discussed in above explanation, but a ceramic filter, combining a plurality of ceramic resonators, will be explained below.
FIG. 4 shows a structural example of a ceramic filter. In this figure, 8-1 and 8-2 are ceramic resonators and C is a capacitor.
As an energy trapping ceramic resonator, to be used as a ladder type ceramic filter 9 shown in FIG. 4, thickness-extensional resonator and thickness-shear resonators utilizing the thickness vibration of a platelet are usually employed. For example, regarding the abovementioned thickness-extensional ceramic resonator, shown in FIG. 2, where the thin film electrodes 72, 72' are attached to both sides of the piezoelectric ceramic platelets 71 by evaporation in the thickness of t; the peripheral edge of piezoelectric the ceramic platelet 71 is held by a metal holding means 74, 74'; said thin film electrodes 72, 72' are connected to the strip electrodes 73, 73'; and a large amount of vibrational energy is trapped within the area sandwiched by thin film electrodes 72, 72', the resonant frequency of such a ceramic resonator is determined by the plate thickness as indicated by equations (2) and (3). EQU f.sub.n =K.sub.E /t.sub.1 .times.n[Hz] (2) EQU f.sub.m =K.sub.S /t.sub.2 .times.m[Hz] (3)
In these equations (2), (3), f.sub.n is the resonant frequency [Hz] of the thickness-extensional ceramic resonator, f.sub.m is the resonant frequency [Hz] of the thickness-shear ceramic resonator, K.sub.E is a constant [Hz m] determined by the material of the thickness-extensional ceramic resonator, K.sub.S is a constant [Hz m] determined by the material of the thickness-shear ceramic resonator, t.sub.1 is thickness [m] of the thickness extensional ceramic resonator, t.sub.2 is thickness [m] of the thickness-shear ceramic resonator, n and m are the degree (1, 3, 5, . . . ).
Namely, the resonant frequency of thickness-extensional ceramic resonator and thickness-shear resonator is inversely proportional to the thickness of plate.
In addition to such thickness-extensional ceramic resonators and thickness-shear ceramic resonators, the width-extensional resonator 8, shown in FIG. 3, which has alleviated the restriction on the selection of the dimension ratio of piezoelectric ceramic platelets and the selection of supporting means of the above resonators, is also considered as an energy trapping ceramic resonator 7 for use in the ladder type ceramic filter shown in FIG. 2.
Namely, existing ceramic filter have employed thickness-extensional ceramic resonator 8-1 or thickness-shear ceramic resonators 8-2 as the ceramic resonators 8 shown in FIG. 4.
However, existing ceramic filters have the following drawbacks. Existing ceramic filters utilizing the above thickness-extensional ceramic resonators, thickness-shear ceramic resonator and width-extensional ceramic resonators having the same plate thickness as the ceramic resonators 8-1 and 8-2 shown in FIG. 4. Therefore, if the main vibration of a ceramic resonator (for example, the thickness-shear basic wave) is used, the resonant frequency of each ceramic resonator becomes equal even in the high order mode (the 3rd, 5th mode of thickness-shear viabration), and the spurious response A appears in the attenuation band as shown in FIG. 5, deteriorating the out-band frequency loss characteristic. Namely, since the signal is not sufficiently lost in the attenuation band, an unwanted signal also exists, lowering the S/N ratio of the filter output signal and causing the performance of the communication equipment to be degraded. As a result, a ceramic filter is required to provide a circuit for suppressing spurious response A in the attenuation band of the output stage of the ceramic filter of FIG. 4, resulting in a disadvantage that the communication equipment becomes large in size.
Such a ceramic filter, having the structure combining plurality of these ceramic resonators, will be further explained from a different viewpoint.
FIG. 6A is an equivalent circuit of the ceramic filter shown in FIG. 4.
In this case, the equivalent circuit of a ceramic resonator can be indicated by the circuit of FIG. 6B. In this figure, Cd is, for example, the damped capacitance of the piezoelectric ceramic platelet 81 of FIG. 3 as the dielectric material, and a series circuit of Lm-Rm-Cm indicates electrical energy and mechanical energy generated by vibration. Namely, an equivalent circuit, shown in FIG. 6B, can be obtained through the generation of width-extensional vibration by applying an AC electric field across the electrodes 82, 82'.
However, the configuration of a ceramic filter using an existing ceramic resonator is always accompanied with following disadvantages.
Namely, a ceramic filter is generally expressed with an equivalent circuit shown in FIG. 6A, but the configuration of a ceramic filter indicated by the equivalent circuit shown in FIG. 6A using a ceramic resonator shown in FIG. 3 requires three capacitors corresponding to C.sub.s1, C.sub.s2, C.sub.c in the equivalent circuit shown in the figure, thus resulting in increase of number of component elements and resultant physical enlargement.
Moreover, a ceramic resonator, shown in FIG. 3, also provides the disadvantage that manufacturing and adjustment require highly accurate grinding work, requiring a long time and a resulting loss in economization. For this reason, if a ceramic filter is configurated using such a ceramic resonator, it becomes considerably expensive.